36 Handbook of Food Science and Technology 3
defined temperature conditions. A powder is assumed to be non-hygroscopic if
its hygroscopicity percentage is less than 10%. The hygroscopicity of a milk
powder is determined by the hydrophilic nature of the components (mainly
lactose and amorphous minerals). Reducing particle size,
by increasing the
powder surface in contact with air of controlled relative humidity, promotes
water adsorption and the hygroscopicity of the powder.
Flowability and floodability
The ability of a powder to flow has a significant impact on storage,
discharge, weighing, mixing, compression, transfer and so on. Carr’s method
[CAR 65] is used to determine two types of behavior: flowability and
floodability. Flowability involves measuring the angle of repose, the angle of
spatula, cohesion and compressibility. Floodability involves measuring the
angle of fall, the angle of difference, dispersibility in the air and the value of
the flowability index. The two main factors affecting the flowability of
powders are particle size distribution and the state of the particle surface.
Powders produced using spray nozzles have a
higher level of flowability
compared to powders produced using heated rollers. Two-stage drying also
yields better flow results compared to single-stage drying. Other factors that
improve flowability are powder agglomeration, a low level of fines, the
addition of a flow agent (silica), the addition of hygroscopic compounds
(carbohydrates, whey) and low levels of free fats.
1.3.3.3.
Technological properties of milk powders
Rehydration properties
The ability of a milk powder to rehydrate in water is an essential property
for industrial users of dried ingredients and can
be characterized by three
properties: wettability, dispersibility and solubility. They depend on powder
composition and the affinity between these components and water, and the
accessibility of water in terms of structure (porosity and capillarity) to the
powder components.
Wettability, the ability of a powder to immerse itself once placed on the
surface of water, reflects the capacity of powder
to absorb the water on its
surface. The swelling ability (swellability) of a powder is also linked to
wettability. The structure of a powder disappears when the various
From Milk to Dairy Products 37
components (in particular proteins) are dissolved or dispersed. Factors
influencing wettability include:
– the presence of large primary particles, such as agglomerated particles:
this is a desired effect with the granulation (with or without recycling fines) of
milk powders;
– powder density;
– the presence of fat on the surface of powder particles (free fats);
– porosity and capillarity of powder particles as well as the presence of
interstitial air.
Dispersibility is probably the best individual criterion for assessing the
rehydration ability of a milk powder, since to a certain extent, it is influenced
by wettability and solubility. Dispersibility is improved by:
– a decrease in protein content;
– an optimal particle size of 200 µm;
– drying at low temperatures (low heat powder).
The insoluble materials formed during the production of milk powder are
usually due to the denaturation of soluble proteins
and the precipitation of
calcium phosphate. Thus, solubility is particularly influenced by heat
treatment before drying, the viscosity and biochemical composition of the
concentrate, the drying air temperature and the particle size of the powder.
Use of recombined milk in cheese processing
The use of recombined milk from powder is justified for several reasons:
for economic, nutritional, dietary and geographical purposes, and also for
sensory and technical purposes. It allows the transfer of cheese production to
countries where milk
production is insufficient, and where milk production has
a high seasonality (in the case of goat’s or sheep’s milk).
Milk powders used in cheese production must have a level of
microbiological quality that is in compliance with regulations and acceptable
for cheese making. These factors depend on the initial quality of the milk used
and the intensity of the heat treatments during processing into powder, which
are the source of physicochemical changes resulting in reduced coagulation
properties when milk powder is reconstituted with water. In order to meet
38 Handbook of Food Science and Technology 3
these microbiological and technological requirements, HTST (pasteurization at
75°C for 20 s) is recommended prior to drying in order to ensure hygienic
quality while maintaining a high level of coagulation. These recommendations
only apply if the milk is of good microbiological quality. Otherwise, the
intensity of the
heat treatment must be higher, therefore compromising
coagulation properties: in this case, the milk powders obtained cannot be used
in the production of cheese.
Cross-flow microfiltration (1.4 µm) followed by vacuum evaporation at
low temperature and spray drying is well suited for the production of an “ultra-
low-heat” powder (low level of denaturation of soluble proteins); milk
reconstituted from this powder, according to regulatory microbiological
requirements, has the same level of rennet coagulation as the original raw milk
([SCH 94], Figure
1.16).
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